a question about gas

[Nick Clifton]

Hi Lope,

> I've uploaded a test case as you requested, there is
> an acompayining README, and some other comments within
> common.S

Phew - next time I will ask for a *simple* test case.

Anyway I fixed the problems with the test (it does not build on an x86_64 host 
for example...) and I traced the source of the problem - you are expecting the 
same assembler source code to be able to access a symbol's value regardless of 
whether the code is compiled for a static library or a shared library.  This is 
not true.  Shared libraries have a more complicated process for accessing a 
variable's value because of the fact that the symbol's address is not known 
until run time.

To see how it should be done, have a look at what the compiler does, eg:

   % cat foo.c
   extern int foo; int bar { return foo; }

   % gcc -S foo.c -o foo.s.static
   % gcc -S -fPIC foo.c -o foo.c.shared
   % cat foo.s.static
   [...]
   bar:
         pushl   %ebp
         movl    %esp, %ebp
         movl    foo, %eax
   [...]
   % cat foo.s.shared
   [...]
   bar:
         pushl   %ebp
         movl    %esp, %ebp
         call    __i686.get_pc_thunk.cx
         addl    $_GLOBAL_OFFSET_TABLE_, %ecx
         movl    foo@GOT(%ecx), %eax
         movl    (%eax), %eax
   [...]

So, this is not an assembler problem but rather a coding problem.   Sorry.


Incidentally with regard to your comment in the common.S about structures, are 
you aware of GAS's .struct pseudo-op ?  You can achieve most of what you want 
with this op, although the problem with accessing global values from shared 
code will still exist.

Cheers
   Nick